1. Field of the Invention
This invention relates to a method and apparatus for transporting fluids at elevated temperatures. More particularly, this invention relates to a method and apparatus for transporting non-inert gaseous fluids or mixtures of inert and non-inert gaseous fluids at elevated temperatures so as to maintain condensable constituents of the non-inert gaseous fluids in a vapor phase during transport.
2. Description of Related Art
When gas is extracted from high temperature processes and conveyed, for example, to a suite of analyzers, it is of great importance that condensation and reaction of any portion of the gas be avoided before it is analyzed. Condensation can be avoided by appropriate dilution with an inert, hot gas, to reduce the concentration of the condensable component, combined with transport through heated, inert tubing. In situations where dilution is impractical, such as when the concentration of trace pollutants of interest, e.g. H2S, COS, or very heavy hydrocarbons such as coronene, end up being below the limit of instrumentation detection, dilution cannot be employed, requiring that the inerted sample lines be maintained as hot as possible.
Sample lines are typically heated by intimate contact with an external sheathed heating element, e.g. heat tracing from about 5/16 in. to about ⅜ in. diameter in lengths of about 80 feet, that has been formed or bent to conform to the sample line and flanges and wired in place, liberally coated with heat-setting, thermally conductive cement and wrapped with heat-resistant foil tape. Thermocouples are usually located near the heaters, typically within the layer of thermally conductive cement, and within the sample line, such as at a “tee” connection, for heater control and measurement of gas temperature. To limit heat loss, external blankets of temperature-resistant insulation are wrapped around the sample transport line and heaters after which the insulation is covered with standard lagging materials that are secured in place. Heater replacement involves removal of all external insulation and insulation lagging, removal of thermocouples, hardened thermal cement, and the foil covering, and uncoiling of the old heater, followed by reinstallation of a new heater element, reapplication of thermal cement, reattachment of the thermocouples, wrapping the assembly with foil tape, and, finally, application and securing of new insulation, if needed, and exterior lagging. Clearly, replacing external heaters that have failed is a labor-intensive process.
In environments that are not intrinsically safe, the surface temperature of external heat tracing and cable heaters cannot exceed about 950° F., the auto-ignition temperature of hydrogen in air. When the energy losses inherent to this approach to heating are taken into account, gas passing through externally heated sample transport lines, connectors, and joints cannot be reliably maintained at more than about 500° F., which is high enough to safely transport acid gases such as HCl or H2S, but not high enough to transport heavy hydrocarbon compounds such as pyrene and coronene. Over time, heavy hydrocarbons accumulated within the sample lines can react, introducing sampling errors, and eventually affecting sample transport. It should be noted that a typical figure for the concentration of hydrocarbon tars (MW≧78) in synthesis gas produced from fluidized-bed biomass gasification is about 10 g/Nm3. Thus, high levels of dilution and/or very high sample gas temperatures may be required to avoid condensation while transporting the synthesis gas to analysis equipment located in a remote, intrinsically safe area.
A further consideration is the efficiency of conventional systems. As previously indicated, current technology employs external heat tracing that is manually wrapped around the sample lines used to convey the sample gas. Because external heaters are employed, even with proper heat insulation and the use of thermally-conductive cements, according to heat tracing manufacturer's representatives, up to about 38% of the heat energy available from these heaters escapes directly to the environment.
In addition to the above-enumerated considerations, external heating is commonly applied to transport tubing that carries inert dilution gas and gas mixtures that are used for calibration and spiking—that is, gases that are injected into a sampling system near the sample extraction point of a process so that in its path to an analyzer, the calibration or spike gas experiences the same environment as that experienced by the sample of gas extracted from the process. Internal tube heaters are known, but only for use in connection with inert fluids. These heaters are essentially coils of nichrome wire or coiled cable heaters that are insulated, e.g. in a quartz tube, and housed within a short section of metal piping in which gas is flowing, maintained at a very high temperature. Usually, these heaters are located very near to the point of use because they are intended only to provide extreme, localized heat.